The present invention relates to a process and an apparatus for detecting a fault in a liquid sheet and in particular in a coating curtain.
In curtain coating processes, especially the coating of photographic compositions, the stability and uniformity of the curtain formed by the liquid composition are important parameters in obtaining an optimum coating quality. Various factors can cause the appearance of faults in the curtain, especially the appearance of waves that propagate in the curtain. A piece of dust on the coating lip or even a wetting fault of the lip or the edge guides of the curtain can cause the appearance of waves in the curtain. Similarly, the presence of the edge guides themselves is a constraint to the natural flow of the curtain that can generate the appearance of waves. These waves propagate up to the contact line with the medium to be coated so that the coating is not uniform. Other faults can appear by the presence of an air current that changes the curtain surface. Such disturbances cause, for example, thickness variations of the composition that are revealed, for example, by the appearance of streaks or lines. Photographic products are very sensitive to these variations. Therefore it is necessary to detect these waves to be able to stop the flow when a fault appears. Traditionally, before starting coating, the coating composition is run and the presence of a wave is observed with the naked eye. Such a method is limited in that it only allows detection of the faults that appear at the start of the flow. The waves that appear during coating, for example, due to the appearance of a piece of dust or any other disturbing item, cannot be detected as the deposition of the photographic layers is done in the dark. In addition, small waves are difficult to see with the naked eye, and yet they cause significant faults to the coating.
U.S. Pat. No. 5,190,789 describes a curtain coating apparatus and process using a photographic composition. During coating, a light or sound wave is sent to a point on the curtain using a transmitter-receiver to check the presence of a curtain, and the reflection of the wave. Measuring the time taken by the reflected wave to reach the transmitter-receiver, especially to check the curvature of the curtain also checks the position of the curtain. Such an apparatus does not enable checking for the presence of a fault that is not found on the entire curtain, like for example, the presence of a wave in the curtain. The wave emitted by the transmitter-receiver is sent to a precise place on the curtain and does not scan over it.
An object of the invention is to provide for a process and an apparatus that enable the fast detection of the presence of a fault in a liquid sheet, especially in a coating curtain. The detection process according to the invention is a reliable process that also enables detection of weakly pronounced faults.
Another object of the invention is to provide for a curtain coating process using a liquid composition wherein the presence of faults is detected in order to stop the flow of the liquid composition as soon as they are noted.
The present invention relates to a detection process of a fault in a flowing liquid sheet. The process comprises the steps of: transmitting an electromagnetic beam onto the sheet in order to irradiate approximately an entire width of the sheet in an approximately transverse direction of the sheet; recovering a beam reflected by the sheet by means of a collector; defining a reference position of the reflected beam on the collector; and noting deviations of the reflected beam in relation to the previously defined reference position.
The present invention also relates to a curtain coating process wherein a composition runs on a surface up to a lip from which the composition flows so as to form a curtain. The coating process comprises the steps of the fault detection process described above and it also comprises the step of stopping the flow of the curtain when a position of the reflected beam on the collector is offset from a set position in relation to the reference position.
The present invention also relates to a detection apparatus for detecting a fault in a flowing liquid sheet comprising: a radiating source for emitting an electromagnetic beam; a shifting device adapted to move the radiating source so that the beam transmitted by the radiating source irradiates approximately the entire width of the sheet in an approximately transverse direction to the sheet; and a collector to recover a beam reflected by the sheet.
The present invention also relates to a detection apparatus for detecting a fault in a flowing liquid sheet comprising: a radiating source for emitting an electromagnetic beam; a slot through which the source irradiates approximately the entire width of the sheet in a direction approximately transverse to the sheet; and a collector to recover a beam reflected by the sheet.